The concept of providing digital sound with cinema film by photographically recording digital audio bits as optical pixels in a stripe along one edge of the film is well-known in the art, as disclosed in U.S. Pat. No. 4,600,280 to Clark. However, such an approach is fraught with perils, including the tendency of the cinema projector to run the film at an unsteady speed and to allow the film to wander laterally across the projector gate. The result is that significant portions of the digital audio bytes photographically recorded on the film are lost or corrupted during playback. This creates unacceptable and distracting noise during the showing of a movie, making the entire concept impractical for commercial use.
This problem was recognized in U.S. Pat. No. 4,839,733 to Karamon et al., in which a solution to the problem is proposed. Karamon et al. propose matching the digital sound track to a conventionally recorded sound track on another portion of the film. If a portion the digital sound track is clearly different from the presumably more reliable conventional sound track, then that portion is discarded and the conventional sound track is used to fill the resulting void. One disadvantage of this approach is that a problem with the digital sound track is not identified until it causes the digital sound track to be at least as bad if not worse in quality than the conventional sound track. Another disadvantage is that, once a problem is identified, the only solution is to temporarily switch to the lower quality conventional sound track, causing a noticeable loss of quality. In many uses, this would be unacceptable. Accordingly, the concept of digital optical sound continues to be mired by problems which at least threaten to make the concept impractical or unmarketable.
Accordingly, there is a great need for a digital optical sound system which automatically corrects for loss of digital audio data caused by movie projector speed fluctuations or by lateral film wander in the projector or by dirt or dust obsuring individual digital pixels on the film.